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Email Protocols Explained

Although it may not seem like it today – in the era of Smartphones, Tablets, and Facebook – I think we can all agree that email is a pretty revolutionary technology. The development of email rendered a whole crop of traditional communications methods all but obsolete.

But why does this matter?

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Even though it’s been around for a while, it’s still every bit as relevant as it used to be. With users accessing email on multiple devices, it is important to understand how email protocols work and allows you to develop an email setup that will make your communications more efficient.

The best part?

We will explain the various protocols and we are here to help you select the perfect email server for hosting. The following are two of the most common questions we get from our customers on the subject of email protocol:

  • Which email protocol is capable of sending emails?

  • What protocol sends email to a mail server?

These answers are so easy that you will wonder why you didn't learn this before. Let’s start with what happens when you receive emails.

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Receiving Email: IMAP vs. POP3

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What Is IMAP?

IMAP (Internet Message Access Protocol) is the email protocol used by the vast majority of consumer email accounts. With IMAP, emails are stored on a remote server. When a user accesses their account through an internet connection, they’re connected to the email provider's servers. The email transfers from the receiving server to the recipient's inbox in their mail client located on their device– which itself doesn’t actually receive any email messages.

One of the key advantages of IMAP – aside from retrieving messages with remote email access – is that it allows a single account to be operated and managed by multiple users, without the need for a complex workaround. It can also put a fair amount of strain on your mail server if your business manages its own email service.

In order for IMAP to function, it requires one of two ports to be open:

  • Port 143: The default IMAP port; this allows the email protocol to listen for incoming requests to synchronize emails. Traffic through this port is a non-encrypted port.

  • Port 993: Used for IMAP over SSL (encrypted communication).

What Is POP3?

Unlike IMAP, POP3 (Post Office Protocol 3) will download messages from a centralized server and transfer them to the recipient email client on their local device or computer. This allows the recipient to download mail to their email client, but also to disconnect from their Internet connection and retain offline access to all of their messages. There are a number of advantages to this approach.

First and foremost, unless you’re opting to leave a copy of each email message on the mail server, it clears up a significant amount of space by storing the messages and files on the user’s local machine. This makes it much easier to back up one’s email messages, in addition to making search and organization far more efficient.

Unfortunately, POP3 isn’t without its weaknesses either. Post Office Protocol 3 by default assumes that only a single user is accessing email from that specific account, meaning that you’ll need a workaround if you want multiple users on a single email account. Finally, the fact that most POP3 services delete downloaded emails from the mail server by default makes it incredibly difficult to synchronize an account to multiple machines.

POP3 requires one of the following as the default port to be open:

  • Port 110: Is the non-encrypted port POP3 access; this is the default listening/communications.

  • Port 995: Used for encrypted POP3 connections.

Which One Should You Use?

And now for the million-dollar question…which of the two email protocols should your business use?

The short answer is IMAP (Internet Message Access Protocol). Given the wealth of different devices typically used by modern employees, the capacity to remotely access one’s email regardless of location is absolutely essential. The longer answer is that – as always – it depends entirely on what you need email for.

The truth is, there’s really nothing stopping you from using multiple email protocols to access an email account – the technology does exist to do so, after all.

Sending Email: SMTP And HTTP

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What Is SMTP?

Though there are two protocols associated with receiving emails, there’s really only one associated with sending them. SMTP (Simple Mail Transfer Protocol) is the gold standard method for passing email messages from one mail server to another. Every email service uses some variation of this protocol when sending mail.

There’s one more difference between SMTP and IMAP/POP3: unlike the latter two, it requires no authentication to function. Although modern SMTP server applications restrict relaying, this nevertheless allows for a fair volume of spam to exist on the web and eventually make it to your email server and thus, your inbox.

The SMTP protocol is also responsible for notifications when an email message arrives. When a sender emails a message, your SMTP server sends commands by the client that specifies the sender's and receiver's email address, along with the entire message and any attachments to the recipient mail server and ensures the protocols fit the request.

There are two ports you’ll need to be aware of for Simple Mail Transfer Protocol client:

  • Port 25: This is the default SMTP port. It is a non-encrypted port.

  • Port 465 / 587: The default port for using SMTP through SSL.

In addition, some hosts may offer a secondary port as an alternative to clients for whom port 25 is filtered.  

SMTP Relay Servers

If you plan to send bulk emails or transactional emails (such as confirmation orders, shipping notifications, or delivery confirmations), you would require the use of a relay server that assists in preventing your emails from becoming spam.

When you are using an email that uses your domain extension (i.e. you@yourcompany.com) and sending to another domain (not internally within your own domain), it doesn't send as quickly because the domain you are using must communicate with the SMTP server and it will relay the information.

SMTP Relay Service From A Third Party

Sending emails in bulk from your domain often will result in them being labeled as spam, which no one wants and can sully your reputation. Therefore it is critical to decide if you plan to set up your own SMTP relay or hire a third party. If you feel like you have a good understanding of how to set up your relay server, then go for it. If not, then you may want to hire a third party.

By doing so you will bypass your domain server so that when sending email campaigns for marketing and sales, not only will you be separating promotional email messages from your internal communications, freeing up system resources on your server, but you will ensure the delivery of your messages are protected. This means the chances of your emails becoming spam are limited and by bypassing your server keeping your data protected.

What Does HTTP Have To Do With Email Protocols?

Now, granted, I’m certain a few of you are at least a little surprised to see HTTP (Hyper Text Transfer Protocol) included on this list. After all, it is not really involved in the process of sending or receiving an email, nor does it directly interact with any of the other mail protocols. So what does it have to do with email protocols, exactly?

Easy – HTTP (and its more secure cousin, HTTPS) is the means by which you’re probably going to access your mailbox, particularly if you’re using a web-based email service like Hotmail or Gmail. For that reason, it actually occupies a fairly important position in the grand scheme of things, even if it’s not technically a mail protocol itself.

The Life-Cycle Of An Email: A Step-By-Step Process

Alright. Now that we’ve explained the protocols behind email, let’s take a look at how they actually come into play and recap a few facts. The process begins when you send email, that email is sent to a remote server tasked with the transport of emails.

The server uses software known as the Mail Transfer Agent (or Message Transfer Agent). The sender’s MTA proceeds to make use of the SMTP server protocol in sending emails to the recipient’s MTA, which itself proceeds to pass on that message to its Mail Delivery Agent on the receiving server. Once the MDA receives the email, it waits for the user to accept it, which is done either through POP3 or IMAP. Upon receipt, it lands in your mailbox where the users receive their mail.

Remember, if you plan to send bulk emails with your domain extension, you want to set up an SMTP server relay.

That’s…a bit confusing, isn’t it?

Let’s simplify things a bit. Perhaps the best analogy I’ve heard in regards to how this whole process function comes courtesy of CCM. which likens MTAs to post offices and MDAs to mailboxes. Makes sense, right?

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In Closing: A Few Additional Details You Should Know About

Now, it’s worth mentioning that there exists a myriad of variations on the POP3 protocol (and by association the IMAP protocol), as well as a number of proprietary email services such as Microsoft’s MAPI. We haven’t mentioned any of those here today because they are not really necessary for an understanding of how email protocols function and how mail servers do their job.

Still, it’s good to be aware of them. Gotta know what your choices are, right?

Are you wondering what a private email server costs?

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Rob Rapiti

Rob Rapiti

Rob is the Content Marketing Manager at Servermania. His goal is to help break down the often complex and technical aspects of IT into relatable, easy-to-understand information.

  • Great article. However, we are quite surprised that you did not mention Port 587. Port 587 is normally used for clients connecting to their ISP email server, and it is effectively SMTP+TLS. In this day and age, security is an absolute must, and while port 25 is still required for server to server communication, all email clients/customers should no longer be using straight POP(110),IMAP(143) or SMTP(25), but use the ‘encrypted’ versions, like IMAP-SSL and SMTP-TLS. This will ensure that their passwords aren’t ‘sniffed’ when sending/receiving email at their local coffee shop, or airport terminal. Also, Port 25 is routinely blocked at many of those locations, so using Port 587 will enable you to still use your favourite email client even at those locations.

  • It is interesting to note that we actually end up using different protocols to send and receive an e-mail. I always had this wrong impression that both send and receive operations can be done by same protocol.